That and MS Office are pretty darn popular. Not the whole world, but a very decent percentage of your users.
If I were to test an email delivery system, I would test Gmail. I probably wouldn't test Google Workspaces, because I'd (wrongly) assume that they work the same.
Maybe the whole thing was intentional, right at the footer of viva "Cloud services by Microsoft Azure" ; #1 I've never heard of viva before #2 I've never seen an azure logo at the footer of a website.